This invention relates to radiation of RF energy from a surface wave transmission line. More specifically, this invention relates to such an arrangement for transmitting and/or receiving RF energy wherein the arrangement exhibits circular polarization relative to the far field radiation pattern and, in addition, exhibits broad band characteristics.
As is known in the art, RF electromagnetic energy will propagate along a single conductor that is configured or treated to concentrate and confine the electromagnetic energy to a cylindrical volume that coaxially surrounds the conductor. This type of transmission line is known as a surface wave transmission line, a Goubau line or a G-line. In the more commonly known surface wave transmission lines, a conductor is surrounded by a coating of low-loss dielectric. Since the phase velocity of the electromagnetic energy that propagates through the dielectric coating is less than the free space phase velocity, at least the majority of the electromagnetic energy is confined to the dielectric and a cylindrical volume of space that concentrically surrounds the dielectric coating. Other techniques for suitably decreasing the phase velocity of the propagating signal also are known. For example, crimping an uncoated wire or machining thread-like grooves in the wire surface will cause a reduction in phase velocity in signals traveling along the wire, thereby causing the uncoated wire to act as a surface wave transmission line.
Since surface wave transmission lines provide a highly efficient transmission medium (low-loss operation) and will support electromagnetic wave propagation over a wide frequency range (broad band operation), application is found in various situations in which environmental conditions can accommodate the unique properties of a traveling surface wave. One such application is the transmission of RF energy along a wire towed by an aircraft such that no intermediate supports are required along the wire which might interfere and cause decoupling of the surface wave energy. One example of an aircraft-surface wave transmission line system that is equipped with means for controlled radiation at or near the end of the wire is disclosed in co-pending U.S. patent application Ser. No. 813,049, now U.S. Pat. No. 4,743,916, filed Dec. 24, 1985 by G. A. Bengeult and entitled "Method and Apparatus For Proportional RF Radiation from Surface Wave Transmission Line." In the system disclosed in the referenced patent application, an electromagnetic wave that is to propagate along the surface wave transmission line is coupled to the transmission line by a rearwardly facing horn-like surface wave "launcher." The launcher in effect serves as a transition between the surface wave transmission line and a coaxial cable or waveguide that serves as a feed line that interconnects the surface wave transmission line with the aircraft RF transmitter or transceiver.
In the radiation system disclosed in the referenced patent application, a series of two or more electrically conductive radiating elements that are spaced-apart by a distance greater than one wavelength (relative to the RF electromagnetic energy that propagates along the surface wave transmission line) are configured in a manner that causes a predetermined portion of the RF electromagnetic energy to become detached from and radiate outwardly from the line. When viewed from the far field, the result is that each radiator appears to be a separate source of radiation.
More specifically, an exemplary arrangement of the type of surface wave transmission line radiation system disclosed in the referenced patent application consists of a conductive conical radiator that is located at the aft terminus of a surface wave transmission line that is towed by an aircraft with the apex of the conical radiator being electrically connected to the transmission line. Located at least one wavelength in front of this radiator is a second radiator (or series of two or more radiators that are spaced apart by at least a wavelength). Each radiator that is located forward of the radiator at the end of the surface wave transmission line is frustoconical in geometry with the outer surface of each such radiator being formed of an electrically conducted material and with the smaller, truncated end of each such radiator facing toward the aircraft. In this arrangement, the surface wave transmission line passes through each frustoconical radiator with the opening that is defined by the smaller, truncated end of the radiator serving as a "window" that allows a predetermined portion of the RF electromagnetic energy that impinges on the radiator to continue propagating toward the end of the transmission line. The remaining portion of the RF electromagnetic wave energy that impinges upon a frustoconical radiator is detached from the line and radiated outwardly into space. Since the electric field (E vector) of the radiated electromagnetic energy is substantially parallel to the surface wave transmission line, the system disclosed in the referenced patent application provides horizontal polarization (with the towed surface wave transmission line considered to be horizontally oriented).
Although a multiple radiator system of this type or a more basic system in which a single electrically conductive conical radiator is attached to the end of a surface wave transmission line fulfills the need for a horizontally polarized, low-loss, broad band RF transmission and radiation system, a need exists for equally broad band and efficient systems that radiate circularly polarized electromagnetic energy. Specifically, in many applications the polarization direction of an antenna that is to receive energy radiated by a surface wave transmission and radiation system either is not known or the antenna (and hence its polarization direction) may change because of movement of the vehicle or structure upon which the antenna is mounted. In such situations and others, polarization mismatch will exist between the radiator used by the surface wave transmission line and the receiving antenna. This means that the electrical signal produced by the antenna will be at a substantially lower level than would be the case if the receiving antenna had the same polarization as the surface wave transmission line. Thus, when polarization mismatch occurs, there is a loss of system efficiency when considered in view of the amount of transmitted energy that is required to produce a desired receive level.
As is known in the art, polarization mismatch in transmitter-receiver situations in which the polarization direction of the receiving antenna is unknown can be eliminated by configuring the transmitting antenna so that the transmitted electromagnetic energy is circularly polarized. When this is done, the electric field factor (E) of the electromagnetic energy that is incident on the receiving antenna rotates in space at a constant angular velocity .omega. radians per second, where .omega.=2.pi.f with f representing the frequency of the transmitted signal in cycles per second. Thus, regardless of the polarization direction of the receiving antenna, maximum electrical coupling will occur once each angular cycle of the E field to thereby result in maximum coupling to the receiving antenna and hence, maximum signal output from the antenna.
Various techniques and antenna construction are known for transmitting circularly polarized electromagnetic energy, with the type and configuration of the transmitting antenna generally depending upon the configuration of the transmission system to be used with the antenna and other factors. In this regard, although the prior art apparently does not address adapting or configuring a surface wave transmission line for radiating circularly polarized electromagnetic energy, one type of circularly polarized antenna that is relevant to the present invention is a center-fed multi-arm spiral antenna. As is known in the art, such spiral antennas include a plurality of antenna arms that are spaced apart from one another and spiral outwardly from associated signal input terminals or feed points that are equally spaced apart from one another along the circumference of a small circle that is located at the center of the antenna. Typically, the conductive antenna arms are mounted on or formed in the surface of a dielectric material that can be planar or of some other geometrical configuration such as conical. Further, it is known that a center-fed multi-arm spiral antenna having N arms or elements is capable of N-1 in dependent modes of operation by suitably establishing the phase difference between the excitation currents that are supplied to the feed points of the antenna arms. In this regard, a first mode of operation (i.e., M= 1) is obtained when the phase different between adjacent feed points (and hence antenna arms) is numeral 2.pi./N. Operation in the M=1 mode is commonly referred to as operation in the "sum" mode and produces a single-lobed radiation pattern that exhibits maximum field strength along, and symmetric about, the antenna boresite axis. Higher order modes (i.e., M=2, 3, . . . (N-1)), often are called "difference" modes and are obtained by feeding the antenna such at the phase difference between adjacent arms is 2.pi.M/N. Operation in a difference mode produces a radiation pattern that exhibits a null along the antenna boresite and maximum field strength along a cone of revolution about the boresite. In this respect, as the mode number increases a larger cone angle is exhibited between the imaginary line of maximum field strength and the antenna boresite axis and a decrease in relative field strength is exhibited.
Although circular polarization has been satisfactorily achieved in many situations by utilizing spiral antennas or other arrangements, the prior art has not yielded a satisfactory circularly polarized radiation arrangement for situations in which the electromagnetic energy being radiated travels along a surface wave transmission line (i.e., situations in which the antenna feed line is a surface wave transmission line). Since, as previously noted, a surface wave transmission line provides a low-loss broad band signal transmission medium, need exists for an arrangement that can be fed from such a surface wave transmission line for radiation of a circularly polarized signal. This is especially true of arrangements such as the type of previously mentioned surface wave transmission line-radiation system that is towed by an aircraft where efficient communication is necessary or desired relative to receiving antennas that exhibit an unknown or variable polarization direction.